Seabairn

The seabairn is a species of aquatic animal that resembles a giant barnacle, with the front half of its body consisting of grey, conical, pointed jaws, like the beak of a squid. The back half of its body is a rough and rocky calcified surface that is roughly conical in shape as well. It has four "legs" which look like squid tentacles mixed with geoduck "necks" - rough, ridged, and dark grey in color. It uses them to propel itself through water in a slightly-spinning motion.
Seabairns are notorious for charging into ships, piercing their hulls with their beaks, and cementing themselves to the hull. From here, their front "leg" tentacles reach inside the ship and flail about, dragging food into its open jaws, which open in three sections triangularly. Its hind tentacles either retract into its carapace to help cement itself, or can be extended to the back of the body to defend itself and influence movement of the vessel it attacked.
Seabairn spawn resembles barnacles, which in themselves is a nuisance and a danger to ships as well as docks, swimmers, and marine life. The barnacles can get caught in jellyfish bodies, killing the jellyfish and causing them to float to the surface, known as a "floating flail," as sailing through these can tear up a ship and cause barnacle attachment which slows vessels and eats away at the hulls.
Their growth is indefinite, so some seabairn can reach a formidable size. Seabairns have adapted to seek out and attack ships, attracted to the wood of the ship and its cargo.
To defend against seabairns, some ships have added iron hulls, although this is expensive and reduces the speed of the boat. Metal, high-powered harpoons have been effective at times against the seabairn, but depending on the angle and spot where it hits, as the seabairn's rocky back half or smooth shell-like front half are quite defensible. Fortunately, seabairns are not the fastest swimmers, so ships travelling at full speeds don't have to worry too much. In slower speeds, however, a seabairn attack could mean disaster for the ship.